Enhancement: Allow specifying limit of `snapshots` list

The `--last` option allowed limiting the output of the `snapshots`
command to the latest snapshot for each host. The new `--latest n`
option allows limiting the output to the latest `n` snapshots.

This change deprecates the option `--last` in favour of `--latest 1`.

https://github.com/restic/restic/pull/3167
